This invention pertains to measuring and detecting devices, but more specifically, to an electronic rate averaging circuit and method therefor which are useful for accurately and reliably determining the average rate of a periodic signal comprising a series of pulses.
Certain critical applications of rate averaging detection demand a very high degree of accuracy and reliability without extensive circuitry, costs and/or power consumption. One such application is in the control of timing operations of functional components of implanted medical devices, such as implantable cardioverters and cardiac pacers which automatically treat arrhythmic heart conditions as they arise in a potentially sick heart of a patient. The components of the implanted medical device are activated on the basis of the rhythm, e.g., timing of electrical events, of the spontaneous beats of the heart. A method or circuit which inaccurately or unreliably measures the spontaneous rhythm might fail to activate the implanted device when it should or, alternatively, activate the implanted device when it should not. In either case, the improper measurement of cardiac events would be undesirable.
Accuracy and reliability can be attained with built-in redundancy or the use of complex circuitry, but this solution usually is costly and impractical for use in implanted medical devices of the type just described where available power and physical space are limited. One previous technique for measuring an average rate employs a resistance-capacitance network which measures the time between the occurrences of electrical events. A drawback of the R-C network is that it is temperature sensitive, e.g., subject to drift in accuracy, and therefore is unreliable. Furthermore, it must be "trimmed" prior to being placed in service, and oftentimes requires trimming thereafter in order to maintain calibration. Obviously, this is undesirable.
Irregularities in periodicity sometimes occur in a periodic signal being measured. This is particularly characteristic of cardiac signals where, to inhibit "false alarms" in an implanted cardioverter, a delay circuit is interposed between the rate sensing circuit and the cardioverter decision circuit to moderate the irregularity. Momentary tachycardia (10-12 beats) is one example of an irregularity which might accidently be interpreted by the automatic cardioverter circuit as a life-threatening arrhythmia, in which case a needless high-energy cardioverting pulse could be delivered to the patient's heart. The delay circuit shields the irregular pulses from the decision circuit, but if the irregularity is sustained, then the irregularity is passed to the decision circuit for appropriate action.
Not only does the environment of an implanted medical device demand high reliability, accuracy, and impose operating limitations on electronic rate averaging circuits, but other applications do so as well. For example, tachometers or inaccessible remote electrical systems may require the same degree of reliability and accuracy, thus the application of the invention described below is not limited to implanted medical devices.